Switching power source apparatus

ABSTRACT

A switching power source apparatus can reduce the size of a transformer and realize the zero-voltage switching of a switch. The apparatus is compact, highly efficient, and low in noise. The apparatus has a series circuit connected to each end of a DC power source (Vdc 1 ) and including a primary winding ( 5   a ) of a transformer (T) and a main switch (Q 1 ), a rectifying-smoothing circuit to rectify and smooth a voltage that is outputted from a secondary winding ( 5   b ) when the main switch (Q 1 ) is turned on, a series circuit connected to each end of the primary winding ( 5   a ) and including an auxiliary switch (Q 2 ) and a clamp capacitor (C 1 ), a series circuit connected to each end of the main switch (Q 1 ) and including a diode (Dx 1 ) and a snubber capacitor (Cx), a series circuit connected to a node between the diode (Dx 1 ) and the snubber capacitor (Cx) and a node between the auxiliary switch (Q 2 ) and the clamp capacitor (C 1 ) and including an auxiliary winding ( 5   x ) and a diode (Dx 2 ), and a control circuit ( 10 ) to alternately turn on/off the main switch (Q 1 ) and auxiliary switch (Q 2 ). When the main switch (Q 1 ) is turned on, the snubber capacitor (Cx) is discharged through the auxiliary winding ( 5   x ) to the clamp capacitor (C 1 ). When the main switch (Q 1 ) is turned off, the snubber capacitor (Cx) is charged, to relax the inclination of a voltage increase of the main switch (Q 1 ).

TECHNICAL FIELD

The present invention relates to a switching power source apparatus thatis highly efficient, compact, and low noise.

FIG. 1 is a circuit diagram showing a switching power source apparatustype according to a related art (Non-Patent Document 1 and Non-PatentDocument 2). In the switching power source apparatus of FIG. 1, a DCpower source Vdc1 is connected through a primary winding 5 a (the numberof turns of n1) of a transformer T to a main switch Q1 that is made of,for example, a MOSFET (hereinafter referred to as FET). Each end of theprimary winding 5 a is connected to a parallel circuit composed of aresistor R2 and a capacitor C2 and a diode D3 that is connected inseries with the parallel circuit. The main switch Q1 is turned on/off byPWM control of a control circuit 100.

The primary winding 5 a and a secondary winding 5 b of the transformer Tare wound to generate in-phase voltages. The secondary winding 5 b (thenumber of turns of n2) is connected to a rectifying-smoothing circuitcomposed of diodes D1 and D2, a reactor L1, and a capacitor C4. Therectifying-smoothing circuit rectifies and smoothes a voltage(ON/OFF-controlled pulse voltage) induced by the secondary winding 5 bof the transformer T and outputs a DC voltage to a load RL.

The control circuit 100 has an operational amplifier and a photocoupler(not shown). The operational amplifier compares an output voltage of theload RL with a reference voltage, and if the output voltage of the loadRL is equal to or larger than the reference voltage, narrows the ONwidth of a pulse to be applied to the main switch Q1. Namely, narrowingthe ON width of a pulse to the main switch Q1 when the output voltage ofthe load RL becomes equal to or larger than the reference voltagecontrols the output voltage to a constant voltage.

Next, operation of the switching power source apparatus having theabove-mentioned configuration will be explained with reference to atiming chart shown in FIG. 2. In FIG. 2, there are shown a terminalvoltage Q1 v of the main switch Q1, a current Q1 i passing to the mainswitch Q1, and a Q1-control signal to conduct ON/OFF control of the mainswitch Q1.

At time t31, the main switch Q1 turns on in response to the Q1-controlsignal, and the DC power source Vdc1 passes the current Q1 i through theprimary winding 5 a of the transformer T to the main switch Q1. Thiscurrent linearly increases as time passes up to time t32. Like thecurrent Q1 i, a current n1 i passing through the primary winding 5 alinearly increases as time passes up to time t32.

Between time t31 and time t32, the main switch Q1 side of the primarywinding 5 a is a negative side “−” and the primary winding 5 a andsecondary winding 5 b are in-phase. Accordingly, the anode side of thediode D1 becomes a positive side “+” to pass a current in order of 5 b,D1, L1, C4, and 5 b.

Next, at time t32, the main switch Q1 changes from ON state to OFF stateaccording to the Q1-control signal. At this time, excitation energy ofthe primary winding 5 a of the transformer T and energy of a leakageinductance Lg (an inductance not coupled with the secondary winding 5 b)are not transferred to the secondary winding 5 b, and therefore, areaccumulated through the diode D3 in the capacitor C2.

Between time t32 and time t33, the main switch Q1 is OFF, and therefore,the current Q1 i and the current n1 i passing through the primarywinding 5 a become zero. Between time t32 and time t33, a current passesin order of L1, C4, D2, and L1, to supply power to the load RL.

According to this switching power source apparatus, insertion of thesnubber circuit (C2, R2) relaxes a temporal change of the voltage of themain switch Q1 to reduce switching noise and suppresses a surge voltageapplied from the leakage inductance Lg of the transformer T to the mainswitch Q1.

Non-Patent Document 1: Kousuke Harada “Switching Power Source Handbook,”Nikkan Kogyo Shinbunsha Shuppan, Chapter 2 Basic Circuit and DesigningPractice of Switching Power Source, p. 27, FIG. 2.2

Non-Patent Document 2: Kazuo Shimizu “High-Speed Switching Regulator,”Sougou Denshi Shuppansha, 2.2.1 Separately Excited Converter, p. 30,FIG. 2.5

DISCLOSURE OF INVENTION

According to the switching power source apparatus shown in FIG. 1,charge accumulated in the capacitor C2 is consumed by the resistor R2,to increase a loss. This loss is proportional to a conversion frequency.If the conversion frequency is increased to reduce the size of theapparatus, the loss increases to deteriorate efficiency.

As shown in FIG. 4, a transformer exciting current passed to the primarywinding 5 a of the transformer T linearly increases at positive valueswhen the main switch Q1 is ON, and when the main switch Q1 is OFF,linearly decreases to zero. Namely, magnetic flux of the transformer Tonly uses the first quadrant of a B-H curve shown in FIG. 3. Thisresults in decreasing the rate of use of a core of the transformer T andincreasing the size of the transformer T.

The present invention provides a switching power source apparatus thatis small, highly efficient, and low noise, allows the size of atransformer to be reduced, and realizes zero-voltage switching.

In order to achieve the object, the present invention of claim 1 is aswitching power source apparatus comprising a first series circuitconnected to each end of a DC power source and including a primarywinding of a transformer and a main switch those are connected inseries, a rectifying-smoothing circuit to rectify and smooth a voltagethat is output from a secondary winding of the transformer when the mainswitch is turned on, a second series circuit connected to each end ofthe main switch or to each end of the primary winding of the transformerand including an auxiliary switch and a clamp capacitor those areconnected in series, a third series circuit connected to each end of themain switch and including a first diode and a snubber capacitor thoseare connected in series, a fourth series circuit connected to a nodebetween the first diode and the snubber capacitor and a node between theauxiliary switch and the clamp capacitor and including an auxiliarywinding of the transformer and a second diode those are connected inseries, and a control circuit to alternately turn on/off the main switchand auxiliary switch. When the main switch is turned on, the snubbercapacitor is discharged through the auxiliary winding to the clampcapacitor, and when the main switch is turned off, the snubber capacitoris charged, to relax the inclination of a voltage increase of the mainswitch.

According to the present invention of claim 2, the control circuit turnson the auxiliary switch to saturate a core of the transformer andincrease an exciting current, and then, turns off the auxiliary switchto make the main switch conduct zero-voltage switching.

According to the present invention of claim 3, the rectifying-smoothingcircuit has a fifth series circuit including the secondary winding and atertiary winding of the transformer, a sixth series circuit connected toeach end of the fifth series circuit and including a first rectifyingdiode and a smoothing capacitor, and a second rectifying diode connectedto a node between the secondary winding and the tertiary winding and anode between the first rectifying diode and the smoothing capacitor.

According to the present invention of claim 4, the primary and secondarywindings of the transformer are wound around the core of the transformerso as to provide a leakage inductance, the primary and tertiary windingsof the transformer are wound so as to provide a leakage inductance thatis smaller than the leakage inductance provided by the primary andsecondary windings, and the primary and auxiliary windings of thetransformer are wound so as to provide a leakage inductance that issmaller than the leakage inductance provided by the primary andsecondary windings and larger than the leakage inductance provided bythe primary and tertiary windings.

According to the present invention of claim 5, a magnetic path of thecore of the transformer has a portion with reduced cross-sectional area.

The present invention of claim 6 is a switching power source apparatuscomprising a first series circuit connected to each end of a DC powersource and including a primary winding of a transformer and a mainswitch those are connected in series, a rectifying-smoothing circuit torectify and smooth a voltage that is output from a secondary winding ofthe transformer when the main switch is turned off, a second seriescircuit connected to each end of the main switch or to each end of theprimary winding of the transformer and including an auxiliary switch anda clamp capacitor those are connected in series, a third series circuitconnected to each end of the main switch and including a first diode anda snubber capacitor those are connected in series, a fourth seriescircuit connected to a node between the first diode and the snubbercapacitor and a node between the auxiliary switch and the clampcapacitor and including an auxiliary winding of the transformer and asecond diode those are connected in series, and a control circuit toalternately turn on/off the main switch and auxiliary switch. Thesnubber capacitor is discharged through the auxiliary winding to theclamp capacitor when the main switch is turned on, the clamp capacitoris discharged through the secondary winding to the rectifying-smoothingcircuit when the auxiliary switch is turned on, and the snubbercapacitor is charged when the main switch is turned off, to relax theinclination of a voltage increase of the main switch.

According to the present invention of claim 7, the rectifying-smoothingcircuit has a series circuit connected to each end of the secondarywinding of the transformer and including a rectifying diode and asmoothing capacitor.

According to the present invention of claim 8, the primary winding andsecondary winding of the transformer are wound around a core of thetransformer to provide a leakage inductance, and the primary winding andauxiliary winding of the transformer are wound to provide a leakageinductance that is larger than the leakage inductance provided by theprimary winding and secondary winding.

As explained above, the present invention provides a low-noise,high-efficiency switching power source apparatus capable of achievingzero-voltage switching and making the rise and fall of a voltage gentlerdue to a resonance action.

The present invention can improve the rate of use of magnetic flux ofthe core of a transformer, to thereby reduce the size of thetransformer. By adjusting the capacitance of the snubber capacitor, thepresent invention can relax the inclination of a voltage increase whenthe main switch is turned off and can adjust the rate of use of magneticflux of the core of the transformer. The energy of the snubber capacitoris discharged to the output side. This results in reducing the noise andsize of the switching power source apparatus and improves the efficiencythereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a switching power source apparatusaccording to a related art.

FIG. 2 is a timing chart showing signals at several parts of theswitching power source apparatus of the related art.

FIG. 3 is a view showing the B-H characteristics of a transformerarranged in the switching power source apparatus of the related art.

FIG. 4 is a timing chart showing an exciting current of the transformerarranged in the switching power source apparatus of the related art.

FIG. 5 is a circuit diagram showing a switching power source apparatusaccording to an first embodiment.

FIG. 6 is a timing chart showing signals at several parts of theswitching power source apparatus of the first embodiment.

FIG. 7 is a timing chart showing the details of the signals at theseveral parts of the switching power source apparatus of the firstembodiment when a switch Q1 is turned on.

FIG. 8 is a timing chart showing the details of the signals at theseveral parts of the switching power source apparatus of the firstembodiment when the switch Q1 is turned off.

FIG. 9 is a view showing a changing state of rise time with respect tothe capacitance of a snubber capacitor Cx in the switching power sourceapparatus of the first embodiment when the switch Q1 is turned off.

FIG. 10 is a view showing the B-H characteristics of a transformer witha low magnetic permeability of core μ in the switching power sourceapparatus of the first embodiment.

FIG. 11 is a view showing the B-H characteristics of a transformer witha high magnetic permeability of core μ in the switching power sourceapparatus of the first embodiment.

FIG. 12 is a timing chart showing a current passing to the transformerarranged in the switching power source apparatus of the firstembodiment.

FIG. 13 is a circuit diagram showing a switching power source apparatusaccording to an second embodiment.

FIG. 14 is a schematic view showing a transformer arranged in theswitching power source apparatus of the second embodiment.

FIG. 15 is a circuit diagram showing a switching power source apparatusaccording to an third embodiment.

FIG. 16 is a schematic view showing a transformer arranged in theswitching power source apparatus of the third embodiment.

FIG. 17 is a timing chart showing signals at several parts of theswitching power source apparatus of the third embodiment.

FIG. 18 is a timing chart showing the details of the signals at theseveral parts of the switching power source apparatus of the thirdembodiment when a switch Q1 is turned on.

FIG. 19 is a timing chart showing the details of the signals at theseveral parts of the switching power source apparatus of the thirdembodiment when the switch Q1 is turned off.

BEST MODE OF IMPLEMENTATION

Switching power source apparatuses according to embodiments of thepresent invention will be explained in detail with reference to thedrawings.

First Embodiment

A switching power source apparatus according to the first embodimentarranges an auxiliary winding for a transformer. When a main switch isturned on, a snubber capacitor connected through a diode is dischargedthrough the auxiliary winding to a clamp capacitor. When the main switchis turned off, the snubber capacitor is charged to make the inclinationof a voltage increase gentle at the time when the main switch is turnedoff. At the same time, an auxiliary switch connected in series with theclamp capacitor is turned on, to bias magnetic flux of the transformertoward a minus side to expand the range of change of magnetic flux andthe auxiliary switch is turned off to make the main switch conductzero-voltage switching (ZVS) when an exciting current increases. In thisway, this embodiment establishes ZVS, improves efficiency, decreasesnoise, and reduces the size of the transformer.

FIG. 5 is a circuit diagram showing the switching power source apparatusaccording to the first embodiment. In the switching power sourceapparatus of FIG. 5, each end of a DC power source Vdc1 is connected toa series circuit consisting of a primary winding 5 a (the number ofturns of n1) of a transformer T1 and a switch Q1 (main switch) made of aFET. Each end of the switch Q1 is connected to a diode D4 in parallel.

A node between a first end of the primary winding 5 a of the transformerT1 and a first end of the switch Q1 is connected to a first end of aswitch Q2 (auxiliary switch). A second end of the switch Q2 is connectedthrough a clamp capacitor C1 to a positive electrode of the DC powersource Vdc1. The second end of the switch Q2 may be connected throughthe clamp capacitor C1 to a negative electrode of the DC power sourceVdc1.

Each end of the switch Q1 is connected to a series circuit consisting ofa diode Dx1 and a snubber capacitor Cx. A node between the diode Dx1 andcapacitor Cx and a node between the switch Q2 and clamp capacitor C1 areconnected to a series circuit consisting of an auxiliary winding 5 x(thenumber of turns of nx) of the transformer T1 and a diode Dx2. Theauxiliary winding 5 x of the transformer T1 discharges energyaccumulated in the capacitor Cx to the clamp capacitor C1 when theswitch Q1 is turned on. By adjusting the capacitance of the capacity Cx,the inclination of a voltage increase of the switch Q1 with respect totime variation is relaxed when the switch Q1 is turned off.

Each end of the switch Q2 is connected to a diode D3 in parallel. Thediode D4 may be a parasitic diode of the switch Q1, and the diode D3 maybe a parasitic diode of the switch Q2. The switches Q1 and Q2 have aperiod (dead time) in which both of them are OFF. These switches arealternately turned on/off under PWM control of a control circuit 10.

The primary winding 5 a and a secondary winding 5 b (the number of turnsof n2) that is in-phase with the primary winding 5 a are wound around acore of the transformer T1. A first end of the secondary winding 5 b isconnected to an anode of a diode D1. A cathode of the diode D1 and asecond end of the secondary winding 5 b are connected to a diode D2.Each end of the diode D2 is connected to a series circuit consisting ofa reactor L1 and a capacitor C4. The diode D1, diode D2, capacitor C4,and reactor L1 form a rectifying-smoothing circuit. The capacitor C4provides a DC output to a load RL.

The control circuit 10 alternately turns on/off the switches Q1 and Q2.If an output voltage of the load RL exceeds a reference voltage, thecontrol circuit 10 narrows the ON-width of a pulse applied to the switchQ1 and widens the ON-width of a pulse applied to the switch Q2. Namely,if an output voltage of the load RL exceeds the reference voltage, theON-width of a pulse to the switch Q1 is narrowed to control the outputvoltage to a constant voltage.

The primary winding 5 a of the transformer T1 usually passes an ACcurrent of equal magnitude. Accordingly, magnetic flux equally increasesand decreases in the first and third quadrants around a zero point on aB-H curve. However, a circuit loss is involved so that magnetic fluxshows incomplete symmetry and is mainly active in the first quadrant.

As a result, by increasing the magnetic permeability μ of the core, thetransformer T1 of the first embodiment produces magnetic flux B (moreprecisely, B is flux density and magnetic flux is φ=B·S where S is thecross sectional area of the core, this embodiment assuming S=1 and φ=B)that saturates at Bm with respect to a given positive magnetic field Has shown in FIG. 11. The magnetic flux also saturates at −Bm withrespect to a given negative magnetic field H. A magnetic field H occursin proportion to the magnitude of a current i. In the transformer T1,magnetic flux B changes along the B-H curve in order of Ba, Bb, Bc, Bd,Be, Bf, and Bg. The changing range of magnetic flux is wide. An intervalbetween Ba and Bb and an interval between Bg and Bf each correspond to asaturated state.

FIG. 10 shows a B-H curve with the magnetic permeability μ of the corebeing low. When the magnetic permeability μ of the core is low, the coredoes not saturate.

Operation of the switching power source apparatus according to the firstembodiment with the above-mentioned configuration will be explained withreference to timing charts of FIGS. 6 to 9 and 12. FIG. 6 is a timingchart showing signals at several parts of the switching power sourceapparatus of the first embodiment. FIG. 7 is a timing chart showing thedetails of the signals at the several parts of the switching powersource apparatus of the first embodiment when the switch Q1 is turnedon. FIG. 8 is a timing chart showing the details of the signals at theseveral parts of the switching power source apparatus of the firstembodiment when the switch Q1 is turned off.

In FIGS. 6 to 8, there are shown a terminal voltage Q1 v of the switchQ1, a current Q1 i passing to the switch Q1, a terminal voltage Q2 v ofthe switch Q2, a current Q2 i passing to the switch Q2, and a terminalvoltage Cxv of the capacitor Cx.

At time t1 (corresponding to time t11 to t12), the switch Q1 is turnedon and a current passes in order of Vdc1, 5 a, Q1, and Vdc1. At thistime, the secondary winding 5 b of the transformer T1 generates avoltage to pass a current in order of 5 b, D1, L1, C4, and 5 b. When theswitch Q1 is turned off, a current passes in order of L1, C4, D2, and L1to supply power to the load RL.

When the switch Q1 is turned on, the primary winding 5 a of thetransformer T1 passes a current n1 i to accumulate energy in an excitinginductance of the transformer T1. As shown in FIG. 12, the current n1 ichanges to take a current value a (negative value) at time t1, a currentvalue b (negative value) at time t1 b, a current value c (zero) at timet13, and a current value d (positive value) at time t2. On the B-H curveof FIG. 11, magnetic flux changes in order of Ba, Bb, Bc, and Bd. Ba toBg of FIG. 11 correspond to a to g of FIG. 12.

At this time, the capacitor Cx is charged to a maximum voltage of theswitch Q1. This voltage and an applied polarity induce a voltage on theauxiliary winding 5 x, and therefore, a current passes in order of Cx, 5x, Dx2, C1, Vdc1, and Cx. As a result, the capacitor Cx entirelydischarges to the clamp capacitor C1, and the voltage Cxv of thecapacitor Cx becomes zero. A current waveform of the capacitor Cx atthis time is determined by a resonance frequency between the capacitorCx and a leakage inductance between the primary winding 5 a andauxiliary winding 5 x of the transformer T1.

Next, at time t2 (t21 to t24), the switch Q1 is turned off, and energyaccumulated in the exciting inductance of the transformer T1 charges thecapacitor Cx through the diode Dx1. At this time, the leakage inductancebetween the primary winding 5 a and auxiliary winding 5 x of thetransformer T1 and the capacitor Cx form a voltage resonance to increasethe voltage Q1 v of the switch Q1.

When the potential of the capacitor Cx becomes equal to the potential ofthe clamp capacitor C1, the diode D3 becomes conductive to pass a diodecurrent to charge the clamp capacitor C1. At this time, the switch Q2 isturned on, so that the switch Q2 becomes a zero-voltage switch. Fromtime t2 to t20, the current n1 i changes from the current value d(positive value) to a current value e (zero). On the B-H curve of FIG.11, magnetic flux changes from Bd to Be.

After the completion of discharge of the energy of the excitinginductance of the transformer T1, the clamp capacitor C1 discharges fromtime t20 to t3 in order of C1, Q2, 5 a, and C1 to reset the magneticflux of the transformer T1.

From time t20 to t3, the energy accumulated in the clamp capacitor C1 isfed back to the primary winding 5 a of the transformer T, and therefore,the current n1 i becomes a negative value as shown in FIG. 12. From timet20 to t2 a, the current n1 i changes from the current value e (zero) toa current value f (negative value). On the B-H curve of FIG. 11,magnetic flux changes from Be to Bf. An area S between time t2 and t20is equal to an area S between time t20 to t2 a. The area S correspondsto the energy of the transformer T1 accumulated in the clamp capacitorC1.

Next, from time t2 a to t3, the current n1 i changes from the currentvalue f (negative value) to a current value g (negative value). On theB-H curve of FIG. 11, magnetic flux changes from Bf to Bg. An area Ebetween time t2 a and t3 corresponds to the energy of the capacitor Cxaccumulated in the clamp capacitor C1.

Namely, the energy accumulated in the clamp capacitor C1 is the sum ofthe energy of the transformer T1 and the energy of the capacitor Cx. Thecurrent n1 i is increased by the energy supplied from the capacitor Cxat the time of resetting. Accordingly, magnetic flux moves to the thirdquadrant to reach the saturation region (Bf-Bg), and the current n1 iincreases to the maximum at time t3 (also at time t1). The current n1 iincreases just before the end of the ON-period of the switch Q2. At thistime, the transformer T1 saturates.

At time t3, the current Q2 i of the switch Q2 also reaches a maximum. Atthis time, the switch Q2 is turned off, and the voltage Q1 v of theswitch Q1 rapidly decreases to zero. Then, the switch Q1 is turned on torealize ZVS of the switch Q1.

The current of the switch Q2 just before turning off the switch Q2 torealize ZVS of the switch Q1 is dependent on the capacitance of thecapacitor connected in parallel with the switch Q1. As the capacitancebecomes smaller, the current becomes smaller. Accordingly, thecapacitance may be set to be small. In a case where the capacitance issmall, however, the inclination of a voltage change increases when theswitch Q1 is turned off, to increase the loss and noise of the switch.To avoid this, it is preferable that the parallel capacitance is smallwhen the switch Q1 is turned on and is large when the switch Q1 isturned off. To realize this, the first embodiment the capacitance isdecreased when turning on the switch Q1 (parasitic capacitance betweenthe drain and source of the switch Q1 will do) and is increased byadding the capacitor Cx in parallel through the diode Dx1 when theswitch. Q1 is turned off.

When the capacitance of the capacitor Cx is sufficiently increased asshown in FIG. 9, the inclination (dv/dt) of a voltage increase of theswitch Q1 with respect to time variation is relaxed when the switch Q1is turned off. This results in reducing the noise and loss of the switchQ1.

Increasing the magnetic permeability μ of the core and enlarging thecapacitance of the capacity Cx transfer the energy of the capacitor Cxto the clamp capacitor C1 and shift the magnetic flux of the transformerT1 to the third quadrant. Those result in expanding the range of use ofthe transformer T1, increasing a current, and easily realizing ZVS ofthe switch Q1. The current becomes larger than that obtained in thesaturation region of the core of the transformer T1, to easily realizethe ZVS operation of the switch Q1.

Second Embodiment

Next, a switching power source apparatus according to the secondembodiment of the present invention will be explained. FIG. 13 is acircuit diagram showing the switching power source apparatus accordingto the second embodiment. The switching power source apparatus of thesecond embodiment of FIG. 13 differs from the switching power sourceapparatus of the first embodiment of FIG. 5 in a circuit on thesecondary side of a transformer T2, and therefore, only this part willbe explained.

The transformer T2 has a primary winding 5 a (the number of turns ofn1), a secondary winding 5 b (the number of turns of n2), and a tertiarywinding 5 c (the number of turns of n3).

Each end of a series circuit consisting of the secondary winding 5 b andtertiary winding 5 c of the transformer T2 is connected to a seriescircuit consisting of a diode D6 and a capacitor C4. A node between thesecondary winding 5 b and the tertiary winding 5 c and a node betweenthe diode D6 and the capacitor C4 are connected to a diode D5. Theprimary winding 5 a and secondary winding 5 b are wound in the samephase, the primary winding 5 a and an auxiliary winding 5 x are wound inthe same phase, and the primary winding 5 a and tertiary winding 5 c arewound in opposite phases.

The secondary winding 5 b and primary winding 5 a of the transformer T2are loosely coupled, and a leakage inductance between the primarywinding 5 a and the secondary winding 5 b is used as a reactor (notshown) connected in series with the transformer T2. The tertiary winding5 c and primary winding 5 a of the transformer T2 are looselymagnetically coupled.

Operation of the switching power source apparatus of the secondembodiment with the above-mentioned configuration will be explained. Abasic operation of the second embodiment is the same as that of thefirst embodiment, and therefore, operation of the circuit on thesecondary side of the transformer T2 will be mainly explained.

When the switch Q1 is turned on, a current passes in order of Vdc1, 5 a,Q1, and Vdc1. At this time, the secondary winding 5 b of the transformerT2 generates a voltage to pass a current in order of 5 b, D5, C4, and 5b. As a result, a current to the diode D5 linearly increases.

Next, the switch Q1 is turned off, and the energy accumulated in theleakage inductance between the primary winding 5 a and secondary winding5 b of the transformer T2 is returned through the transformer T2 to thesecondary side. On the secondary side, a voltage is induced on thetertiary winding 5 c of the transformer T2, to pass a current in orderof 5 c, D6, C4, 5 b, and 5 c. Namely, current passes through the diodeD6.

In this way, the value of the inductance connected in series with theprimary winding 5 a of the transformer T2 is increased to return energyaccumulated when the switch Q1 is ON to the secondary side through thetransformer T2, to thereby increase efficiency. Due to the diodes D5 andD6, a secondary current continuously passes during the ON and OFFperiods of the switch Q1, to reduce a ripple current of the capacitorC4.

FIG. 14 is a view showing the structure of the transformer installed inthe switching power source apparatus of the second embodiment. Thetransformer T2 shown in FIG. 14 has a core 30 having an H-shape withclosed top and bottom. A core part 30 a of the core 30 is wound with theprimary winding 5 a and tertiary winding 5 c those are close to eachother to form a slight leakage inductance between them. The core 30 hasa path core 30 c and a gap 31. A peripheral core is wound with thesecondary winding 5 b. The auxiliary winding 5 x is wound close to theprimary winding 5 a. Namely, the path core 30 c loosely couples theprimary winding 5 a with the secondary winding 5 b, to increase theleakage inductance.

The core 30 of the transformer T2 is wound with the primary winding 5 aand secondary winding 5 b so as to form the leakage inductance, theprimary winding 5 a and tertiary winding 5 c are wound to form a leakageinductance that is smaller than that formed by the primary winding 5 aand secondary winding 5 b, and the primary winding 5 a and auxiliarywinding 5 x are wound to form a leakage inductance that is smaller thanthat formed by the primary winding 5 a and secondary winding 5 b andlarger than that formed by the primary winding 5 a and tertiary winding5 c.

On the peripheral core between the primary winding 5 a and the secondarywinding 5 b, there are formed two recesses 30 b. The recesses 30 bpartly narrow the cross sectional area of a magnetic path of theperipheral core, so that only the narrowed parts saturate to reduce acore loss.

By devising the shape and windings of the core of the transformer T2 insuch a way, this embodiment reduces the switching power source apparatusin size and cost.

Third Embodiment

Next, a switching power source apparatus according to the thirdembodiment of the present invention will be explained. FIG. 15 is acircuit diagram showing the switching power source apparatus accordingto the third embodiment. The switching power source apparatus of thethird embodiment of FIG. 15 differs from the switching power sourceapparatus of the first embodiment of FIG. 5 in a circuit on thesecondary side of a transformer T3, and therefore, only this part willbe explained.

A core of the transformer T3 is wound with a primary winding 5 a and asecondary winding 5 b (the number of turns of n2) that is of an oppositephase relative to the phase of the primary winding 5 a. A first end ofthe secondary winding 5 b is connected to an anode of a diode D1. Acathode of the diode D1 and a second end of the secondary winding 5 bare connected to a capacitor C4. The diode D1 and capacitor C4 form arectifying-smoothing circuit. The capacitor C4 smoothes a rectifiedvoltage of the diode D1 and provides a DC output to a load RL.

FIG. 16 is a view showing the structure of the transformer installed inthe switching power source apparatus of the third embodiment. Thetransformer T3 shown in FIG. 16 has a core 40 having an H-shape withclosed top and bottom. A core part 40 a of the core 40 is wound with theprimary winding 5 a and an auxiliary winding 5 x those are close to eachother to provide a leakage inductance between them. The secondarywinding 5 b is wound in concentric with the primary winding 5 a andauxiliary winding 5 x, to provide a slight leakage inductance. The corepart 40 a has a gap 41.

The core 40 of the transformer T3 is wound with the primary winding 5 aand secondary winding 5 b so as to form a leakage inductance, and theprimary winding 5 a and auxiliary winding 5 x are wound to form aleakage inductance that is larger than that formed by the primarywinding 5 a and secondary winding 5 b.

By devising the shape and windings of the core of the transformer T3 insuch a way, this embodiment reduces the switching power source apparatusin size and cost.

Operation of the switching power source apparatus of the thirdembodiment with the above-mentioned configuration will be explained withreference to timing charts of FIGS. 17 to 19. FIG. 17 is a timing chartshowing signals at several parts of the switching power source apparatusof the third embodiment. FIG. 18 is a timing chart showing the detailsof the signals at the several parts of the switching power sourceapparatus of the third embodiment when a switch Q1 is turned on. FIG. 19is a timing chart showing the details of the signals at the severalparts of the switching power source apparatus of the third embodimentwhen the switch Q1 is turned off.

In FIGS. 17 to 19, there are shown a terminal voltage Q1 v of the switchQ1, a current Q1 i passing to the switch Q1, a terminal voltage Q2 v ofa switch Q2, a current Q2 i passing to the switch Q2, a current Cxipassing to a capacitor Cx, a terminal voltage Cxv of the capacitor Cx,and a current D1 i passing to the diode D1.

The timing charts of FIGS. 17 to 19 are substantially the same as thoseof FIGS. 6 to 8. Only difference between them is operation of thecircuit on the secondary side of the transformer T3, and therefore,operation of only the secondary side will be explained.

At time t1 (corresponding to time t11 to t14), the switch Q1 is turnedon to pass a current in order of Vdc1, 5 a, Q1, and Vdc1. At this time,no current passes to the diode D1.

At time t2 (t21 to t23), the switch Q1 is turned off, so that thecapacitor Cx relaxes the inclination of a voltage increase in the OFFstate of the switch Q1. Energy accumulated in the capacitor Cx issupplied to the secondary winding 5 b of the transformer T3 when theswitch Q2 is turned on to pass the current D1 i to the diode D1 andsupply power to a load RL.

In this way, the switching power source apparatus of the thirdembodiment provides an effect similar to that provided by the switchingpower source apparatus of the first embodiment.

INDUSTRIAL APPLICABILITY

The switching power source apparatuses according to the presentinvention are applicable to DC-DC converting type power source circuitsand AC-DC converting type power source circuits.

1. A switching power source apparatus comprising: a first series circuitbeing connected to each end of a DC power source and including a primarywinding of a transformer and a main switch those are connected inseries; a rectifying-smoothing circuit to rectify and smooth a voltagethat is outputted from a secondary winding of the transformer when themain switch is turned on; a second series circuit being connected toeach end of the main switch or to each end of the primary winding of thetransformer and including an auxiliary switch and a clamp capacitorthose are connected in series; a third series circuit being connected toeach end of the main switch and including a first diode and a snubbercapacitor those are connected in series; a fourth series circuit beingconnected to a node between the first diode and the snubber capacitorand a node between the auxiliary switch and the clamp capacitor andincluding an auxiliary winding of the transformer and a second diodethose are connected in series; and a control circuit to alternately turnon/off the main switch and auxiliary switch, wherein the snubbercapacitor is discharged through the auxiliary winding to the clampcapacitor when the main switch is turned on; and the snubber capacitoris charged when the main switch is turned off, to relax the inclinationof a voltage increase of the main switch.
 2. The switching power sourceapparatus of claim 1, wherein the control circuit is configured to turnon the auxiliary switch to saturate magnetic flux in a core of thetransformer and turn off the auxiliary switch to make the main switchconduct zero-voltage switching as an exciting current increases.
 3. Theswitching power source apparatus of claim 1, wherein therectifying-smoothing circuit has a fifth series circuit including thesecondary winding and a tertiary winding of the transformer, a sixthseries circuit being connected to each end of the fifth series circuitand including a first rectifying diode and a smoothing capacitor, and asecond rectifying diode being connected to a node between the secondarywinding and the tertiary winding and a node between the first rectifyingdiode and the smoothing capacitor.
 4. The switching power sourceapparatus of claim 3, wherein the primary and secondary windings of thetransformer are wound around the core of the transformer so as toprovide a leakage inductance; the primary and tertiary windings of thetransformer are wound so as to provide a leakage inductance that issmaller than the leakage inductance provided by the primary andsecondary windings; and the primary and auxiliary windings of thetransformer are wound so as to provide a leakage inductance that issmaller than the leakage inductance provided by the primary andsecondary windings and larger than the leakage inductance provided bythe primary and tertiary windings.
 5. The switching power sourceapparatus of claim 4, wherein a magnetic path of the core of thetransformer has a portion with reduced cross-sectional area.
 6. Aswitching power source apparatus comprising: a first series circuitbeing connected to each end of a DC power source and including a primarywinding of a transformer and a main switch those are connected inseries; a rectifying-smoothing circuit to rectify and smooth a voltagethat is outputted from a secondary winding of the transformer when themain switch is turned off; a second series circuit being connected toeach end of the main switch or to each end of the primary winding of thetransformer and including an auxiliary switch and a clamp capacitorthose are connected in series; a third series circuit being connected toeach end of the main switch and including a first diode and a snubbercapacitor those are connected in series; a fourth series circuit beingconnected to a node between the first diode and the snubber capacitorand a node between the auxiliary switch and the clamp capacitor andincluding an auxiliary winding of the transformer and a second diodethose are connected in series; and a control circuit to alternately turnon/off the main switch and auxiliary switch, wherein the snubbercapacitor is discharged through the auxiliary winding to the clampcapacitor when the main switch is turned on; the clamp capacitor isdischarged through the secondary winding to the rectifying-smoothingcircuit when the auxiliary switch is turned on; and the snubbercapacitor is charged when the main switch is turned off, to relax theinclination of a voltage increase of the main switch.
 7. The switchingpower source apparatus of claim 6, wherein the rectifying-smoothingcircuit has a series circuit of a rectifying diode and a smoothingcapacitor that is connected to each end of the secondary winding of thetransformer.
 8. The switching power source apparatus of claim 7, whereinthe primary winding and secondary winding of the transformer are woundaround a core of the transformer so as to provide a leakage inductance,and the primary winding and auxiliary winding of the transformer arewound so as to provide a leakage inductance that is larger than theleakage inductance provided by the primary winding and secondarywinding.
 9. The switching power source apparatus of claim 2, wherein therectifying-smoothing circuit has a fifth series circuit including thesecondary winding and a tertiary winding of the transformer, a sixthseries circuit being connected to each end of the fifth series circuitand including a first rectifying diode and a smoothing capacitor, and asecond rectifying diode being connected to a node between the secondarywinding and the tertiary winding and a node between the first rectifyingdiode and the smoothing capacitor.
 10. The switching power sourceapparatus of claim 9, wherein the primary and secondary windings of thetransformer are wound around the core of the transformer so as toprovide a leakage inductance; the primary and tertiary windings of thetransformer are wound so as to provide a leakage inductance that issmaller than the leakage inductance provided by the primary andsecondary windings; and the primary and auxiliary windings of thetransformer are wound so as to provide a leakage inductance that issmaller than the leakage inductance provided by the primary andsecondary windings and larger than the leakage inductance provided bythe primary and tertiary windings.
 11. The switching power sourceapparatus of claim 10, wherein a magnetic path of the core of thetransformer has a portion with reduced cross-sectional area.